Hydra is an effort to simplify the development of interoperable, hypermedia-driven Web APIs. The two fundamental building blocks of Hydra are JSON-LD and the Hydra Core Vocabulary.
JSON-LD is the serialization format used in the communication between the server and its clients. The Hydra Core Vocabulary represents the shared vocabulary between them. By specifying a number of concepts which are commonly used in Web APIs it can be used as the foundation to build Web services that share REST's benefits in terms of loose coupling, maintainability, evolvability, and scalability. Furthermore it enables the creation of generic API clients instead of requiring specialized clients for every single API.
To participate in the development of this specification please join the Hydra W3C Community Group.
More information about Hydra is currently available at: http://www.markus-lanthaler.com/hydra